Resolve "Add error handling in case the paths for gNMI Subscription for an MNE is empty"

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Related to #333

controller/northbound/server/networkElement.go

@@ -9,6 +9,7 @@ import (
 	mnepb "code.fbi.h-da.de/danet/gosdn/api/go/gosdn/networkelement"
 	ppb "code.fbi.h-da.de/danet/gosdn/api/go/gosdn/pnd"
 	tpb "code.fbi.h-da.de/danet/gosdn/api/go/gosdn/transport"
+	"code.fbi.h-da.de/danet/gosdn/controller/config"
 	"code.fbi.h-da.de/danet/gosdn/controller/conflict"
 	"code.fbi.h-da.de/danet/gosdn/controller/customerrs"
 	"code.fbi.h-da.de/danet/gosdn/controller/interfaces/change"
@@ -629,7 +630,9 @@ func (n *NetworkElementServer) addMne(ctx context.Context,
 			return uuid.Nil, err
 		}
 
-		n.networkElementWatchter.SubscribeToNetworkElement(mne, nil)
+		if mne.GetGnmiSubscriptionPaths() != nil || config.GetGnmiSubscriptionPaths() != nil {
+			n.networkElementWatchter.SubscribeToNetworkElement(mne, nil)
+		}
 	} else {
 		err = fmt.Errorf("invalid transport data provided")
 		return uuid.Nil, err